Liquid Crystal Display (LCD) devices are widely used in flat-panel displays. A conventional LCD device is a Thin Film Transistor Liquid Crystal Display (simply referred to as TFT-LCD). A Thin Film Transistor (TFT) includes an amorphous-Silicon Thin Film Transistor (a-Si TFT) and a Poly-Silicon Thin Film Transistor (Poly-Si TFT). Furthermore, the Poly-Silicon Thin Film Transistors includes Low Temperature Poly-silicon (referred to as LTPS) TFT-LCDs and High Temperature Poly-silicon (referred to as HTPS) TFT-LCDs. Specifically, the TFT-LCDs in the prior art are referred to as the a-Si TFT-LCDs. The circuit diagram of the internal structure of a TFT-LCD is shown in FIG. 1, and the structure will be described as follows.
Multiple scanning lines 51 to Sm parallel to each other and multiple data lines D1 to Dn parallel to each other are disposed on the array substrate of the TFT-LCD. The scanning lines are arranged to intersect with the data lines and pixel units 110 are disposed at the intersections of the scanning lines and the data lines to form a pixel unit array. The pixel unit 110 includes a transistor 112 of which the gate is connected to a corresponding scanning line, the source of the transistor 112 is connected to a corresponding data line and the drain of the transistor 112 is coupled to a common voltage Vcom via a pixel element 114. Specifically, a storage capacitor 116 connected across the pixel element 114 to stabilize the voltage across the pixel element 114.
When an image is displayed by the TFT-LCD, a scanning signal is transmitted to the scanning lines to turn on a transistor connected to each of the scanning lines. Then, a data signal is sequentially applied to the pixel units 110, and the storage capacitor 116 will be charged upon the data signal is transmitted from the source to the drain of transistor 112. Meanwhile, the common voltage Vcom is applied to the other end of the pixel element 114, since Vcom is different from the voltage of the drain of the transistor, the voltage difference is formed between the two ends of the pixel element 114, and thus the pixel element 114 is lit up.
On the array substrate of the TFT-LCD, the common voltage is applied to the whole substrate, therefore, when one pixel unit is driven, the common voltage is applied to the whole substrate simultaneously. That is, the load driven by the common voltage Vcom may be all the pixel units on the array substrate, so that the common voltage Vcom may not have sufficient drive capability.
In order to solve the problem, the solution according to the prior art is shown in FIG. 2. One common line is disposed for each row of pixel units, and a common voltage is applied to the pixel units via the common line. The common line is connected to a corresponding scanning line via a transistor 21. Here, the gate of the transistor 21 is connected to the corresponding scanning line, and the drain of the transistor 21 is connected to the common line, and the sources of all the transistors 21 on the array substrate are connected to a terminal from which the common voltage is supplied.
During display of an image, transistors within a row of pixel units and the transistor 21 connected to the corresponding common line are turned on simultaneously by a scanning signal, thus the common voltage is applied to the pixel units row by row, the load for the common voltage is reduced and the drive capability of Vcom is improved.
However, adopting this design will affect the quality of display due to the extremely short charging time of the pixel located away from a common voltage signal input terminal which is caused by the RC delay. Since a common electrode line has the common voltage only when the row of scanning line is turned on, the common electrode line is in a floating state at other times and can be affected by disturbance, and thus the quality of display will be deteriorated.